For data transmission on a standard voice grade telephone line a modem is employed which is operative to modulate data in a form suitable for transmission along the telephone line, and to demodulate data received from the telephone line to reproduce the data transmitted from a remote site. The modem requires a power supply for energizing its electronic circuits and this power supply is usually operative from an AC power outlet or a battery source.
In many instances an AC power source is not conveniently available, and, in the case of a battery source, the modem may be at a location where battery monitoring or replacement is inconvenient. In addition, the power supply adds to the expense and size of a modem. It would be advantageous to provide power for operation of the modem circuits from the telephone line itself to thereby dispense with the need for any separate modem power supply or access to a separate power source.
A modem is disclosed in copending application of O. Leon Pierce, Mark C. Smith, and R. Byron Driver, entitled Line Powered Modem, Ser. No. 203,108 filed Nov. 3, 1980 now U.S. Pat. No. 4,395,590 and assigned to the same assignee as this invention, in which the entire operating power for the modem circuits is derived from the available telephone line power.